Sensor element for limiting current sensors to determine the lambda value of gas mixtures

ABSTRACT

Sensor element for limiting current sensors to determine the lambda value of gas mixtures, in particular of exhaust gases of internal combustion engines, having inner and outer pump electrodes disposed on a ceramic substrate, the inner pump electrode of which is accessible for the measurement gas supplied through a diffusion layer acting as a diffusion barrier; having a gas-tight cover layer above the diffusion layer and having conductor tracks for the pump electrodes is described, in which the diffusion layer is adjustable, to calibrate the sensor element. This adjustability is advantageously attained by embodying the side of the diffusion layer 5 oriented toward the measurement gas inlet opening as a zigzag-shaped adjustment zone 8, from which parts can be separated mechanically or by laser cuts. The invention makes comparatively simple calibration of sensor elements possible.

PRIOR ART

The invention is based on a sensor element for limiting current sensors.In such sensor elements, which operate by the diffusion limiting currentprinciple, the diffusion limiting current is generally measured at aconstant voltage applied to both electrodes of the sensor element. Inexhaust gas produced in combustion processes, current is dependent onthe oxygen concentration, as long as the diffusion of the gas to thepump electrode determines the speed of the electrode reaction takingplace. It is known to structure such sensors, which operate by thepolarographic measuring principle, in such a way that both the anode andthe cathode are exposed to the gas to be measured; the cathode has adiffusion barrier, to enable operating in the diffusion limiting currentrange.

As a rule, the known limiting current sensors serve to determine thelambda value of exhaust gas mixtures, which expresses the ratio betweentotal oxygen and the oxygen required for complete combustion of the fuelin the fuel/air mixture burning in a cylinder.

Because of its simplified and relatively inexpensive manufacture, theproduction of probes and sensor elements that can be made by ceramicfoil and screenprinting techniques has become established in theindustry in recent years.

Planar polarographic probes can be produced simply and economicallybeginning with oxygen-carrying solid electrolytes in the form of smallplates or foils, for instance of stabilized zirconium dioxide, which arecoated on both sides with an (inner or outer) pump electrode and withthe associated conductor track. The inner pump electrode isadvantageously located at the end of a diffusion gap or diffusionchannel, through which the measurement gas can diffuse in, and whichserves as a gas diffusion resistor.

Sensor elements and detectors are also known from German PatentDisclosure Document 35 43 759 and from European Patent Applications 0142 992, 0 142 993, 0 188 900 and 0 194 082, which have in common thefact that they each have a pump cell and a sensor cell, which compriseoxygen-carrying solid electrolytes in the form of small plates or foils,and two electrodes disposed on them, and have a common diffusion gap ordiffusion channel.

German Patent Disclosure Document 38 34 987 also describes a sensorelement for limiting current sensors for determining the lambda value ofgas mixtures, in particular the exhaust gases of internal combustionengines, having a pair of pump electrodes disposed on a solidelectrolyte that conducts O² -ions, in which the inner pump electrodecommunicates with the measurement gas via a diffusion gap, and thediffusion gap is covered by a solid electrolyte layer produced byscreenprinting.

A disadvantage of the known sensor elements, which are in particularmanufactured by laminating a plurality of solid electrolyte foilstogether, particularly those based on stabilized ZrO₂, is that a laterchange in the geometry of the diffusion layer, for instance for the sakeof calibrating the sensor elements once the sensor elements have beenfinally sintered together, is as a rule difficult. The magnitude of thediffusion current in limiting current sensors in fact depends on theshaping of the diffusion layer, which can be made by various methods. Inceramic probes, such diffusion barriers are preferably applied tounsintered ceramic substrates by screenprinting and the entire assemblyis then sintered. Only then can the limiting current be measured.

To preclude this disadvantage, it is known, for instance from EuropeanPatent Application 0 191 627, to use sensor elements having adjustableresistors on the sensor body. However, this has the disadvantage thatmanufacturing such a sensor element is comparatively complicated andexpensive, and that additional electric leads are needed for thispurpose on the sensor body.

ADVANTAGES OF THE INVENTION

The sensor element according to the invention has the substantialadvantage over the element known from European Patent Application 0 191627 that it is simpler to manufacture and that fewer electrical leadsare needed. In the case of a sensor element according to the invention,varying the diffusion resistance of the finished sensor element ispossible within wide limits, by successively expanding the inlet openingof the gas into the diffusion layer mechanically or by means of lasercuts. This is made possible by zigzag contouring of the diffusion layerat its end at which the measuring gas inlet opening is provided. If as aresult the diffusion resistance of a finished sensor element is too highfor it to operate in a favorable load range, then a portion of theadjustment zone is severed mechanically or by laser cuts. The number,length and spacing of the zigzags from one another are selected suchthat the pump electrode can operate in a favorable load range, inaccordance with the quantity of diffused-in measurement gas.

Mechanical severing of parts of the adjustment zone is suitably done insuch a way that the limiting current is set to a defined value duringongoing measurement, by sandblasting or by means of a ceramic saw;because of the mechanical machining, the tunnel inputs, such as thetunnel inputs shown in FIG. 2 formed by the adjustment zone 8 of thediffusion layer 5, are expanded.

When parts of the adjustment zone are severed by laser cuts, theprocedure is equivalent, in that the tunnel inputs are opened by lasercuts until a defined limiting current flows.

The geometry of the diffusion layer is adapted to the given factors inan individual case. This means that the diffusion layer can take verydifferent forms and have quite different measurements.

In the case of rectangular electrodes, for instance, it can cover theelectrodes geometrically similarly, or in the case of round electrodesit can encompass them circularly. The number, length and spacing of thezigzags from one another are selected in such a way, preferably as afunction of the available electrode surface area, that the electrodescan operate in a favorable load range in accordance with the diffused-inquantity of gas.

The sensor element according to the invention can be used in limitingcurrent sensors of the conventional type, instead of known sensorelements of planar structure. Possible examples include wide bandsensors (lambda less than, greater than, or equal to one) and leansensors (lambda>1). The sensor element according to the invention canthus be embodied by itself as a pump element, optionally with a heatingelement, for instance as a lean sensor for Diesel engines, and as suchcan be installed into a typical sensor housing, for instance the typeknown from German Patent Disclosure Documents 32 06 903 and 35 37 051,and used to measure the fuel/air ratio in a lean or rich exhaust gas.However, in addition to the pump cell, the sensor element according tothe invention can additionally have a sensor cell (Nernst cell), whichis provided with an additional air reference channel, and one electrodeof which is disposed in the region of the pump electrode in thediffusion channel of the pump cell, and the other electrode of which islocated in the air reference channel.

DRAWING

Advantageous embodiments of a sensor element according to the inventionare shown by way of example in the drawing. Specifically, shown are in:

FIG. 1, a first embodiment of a sensor element according to theinvention, in a schematic sectional view;

FIG. 2, the shape of a diffusion layer, shown schematically in a planview; and

FIG. 3, a second embodiment of a sensor element, seen in a schematicsectional view.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The sensor element schematically shown in FIG. 1 comprises the ceramicsubstrate 1, onto which the inner pump electrode (cathode) 2 with theconductor track 2' and the outer pump electrode (anode) 3 with theconductor track 3' are applied. The inner pump electrode 2 is suppliedwith measurement gas via the measurement gas inlet 4 and the diffusiongap 6 which is filled with the diffusion layer 5. The pump electrode 2,conductor track 2' and diffusion layer 5 are covered by the gas-tightcover layer 7.

FIG. 2 shows the geometrical shape of the diffusion layer 5 with thezigzag-shaped adjustment zone 8. The dashed lines represent regions thatcan for instance be severed.

The sensor element schematically shown in FIG. 3 comprises the ceramicsubstrate 1, the inner pump electrode 2 with the associated conductortrack 2', the diffusion layer 5, the solid electrolyte layer 9, theouter pump electrode 3 with the associated conductor track 3', theinsulation layer 10, the engobe 11 and the cover layer 7. The entry ofmeasurement gas takes place at 12. The diffusion layer 5 can also have ageometrical shape such as that shown by way of example in FIG. 2.

The porous diffusion layers 5 acting as diffusion barriers can beprinted onto the unsintered ceramic substrate by screenprinting, usingtypical printing pastes. However, it is also possible to printheat-eroded layers onto the unsintered ceramic substrate and to sinterthe entire assembly layer. Foils of suitable geometric shape that sinterin porous fashion can also be used to embody the diffusion layer.

Advantageously, the diffusion layer 5 and the adjustment zone 8 comprisea ceramic material that sinters in a coarsely porous fashion, such asone based on Al₂ O₃ or ZrO₂. The porosity of the diffusion layer can beset by the addition of pore formers, which burn, decompose or evaporatein the sintering process. Typical pore formers that can be used include,for example, thermal soot powder; plastics, for instance based onpolyurethane; salts, such as ammonium carbonate; and organic substances,such as theobromine and indanthrene blue. Such pore formers are added tothe porously sintering starting material in a quantity such that amaterial having a porosity of 10 to 50%, for instance, is produced. Themean pore diameter, which can be determined by the particle size of thepore former used, is preferably in the range of approximately 0.1 to 10micrometers.

Advantageously, the diffusion layer 5 can further be embodied such thatboth Knudsen and gas-phase diffusion take place. This means that thediffusion layer forming the diffusion barrier has a channel system formixed diffusion comprising both Knudsen and gas-phase diffusion, asdescribed in further detail in German Patent Disclosure Document 37 28289.

The ceramic substrate 1 comprises a ceramic material of the kindtypically used to produce sensor elements, for instance based on ZrO₂ orAl₂ O₃. Preferably the ceramic substrate 1 comprises one of the knownoxides, used to make solid electrolyte foils that conduct O²⁻ ions, ofquadrivalent metals, such as ZrO₂, CeO₂, HfO₂ and ThO₂ in particular,having a content of bivalent alkaline earth oxides and/or preferablytrivalent rare earth oxides. Typically, the layer can comprise up to 50to 97 Mol % ZrO₂, CeO₂, HfO₂ or ThO₂ and 50 to 3 Mol % CaO, MgO or SrO,and/or oxides of rare earths and in particular Y₂ O₃. It has provedadvantageous to manufacture the sensor elements in the form of ceramicsubstrates, to use foils of unsintered ceramic material with a layerthickness of from 0.3 to 1.0 mm and in particular approximately 0.5 mm.

The pump electrodes 2 and 3 and the associated conductor tracks 2' and3' preferably comprise a metal of the platinum group, in particularplatinum, or alloys of metals of the platinum group or alloys of metalsof the platinum group with other metals. Optionally, they contain aceramic support structure material, for instance in the form of a YSZpowder, having a volumetric proportion of preferably approximately 40volume %. They are porous and as thin as possible. Preferably, they havea thickness of 8 to 15 micrometers. The conductor tracks belonging tothe pump electrodes preferably likewise comprise platinum or a platinumalloy of the type described. They may also be produced beginning with apaste based on noble metal and cermet.

Pastes suitable for printing the pump electrodes and conductor trackscan be prepared in a known manner, using organic binders and/or adhesionpromoters, plasticizers and organic solvents. If insulating intermediatelayers are to be produced simultaneously as well, then lesser quantitiesof compounds having a pentavalent or higher valent cation, such as Nb₂O₅, can be added to the pastes. Al₂ O₃ or ZrO₂ are suitable asadhesion-promoting additives.

The gas-tight cover layer 7 for instance comprises a layer based on Al₂O₃ or Mg-spinel, as is typically used in planar sensor elements to coverelectrodes

In the case of the embodiment shown by way of example in FIG. 3 of asensor element according to the invention, the solid electrolyte layer 9comprises one of the known oxides, used for producing solid electrolytefoils that conduct O²⁻ ions, of quadrivalent metals, such as ZrO₂, CeO₂,HfO₂ and ThO₂, in particular, having a content of bivalent alkalineearth oxides and/or preferably trivalent rare earth oxides. Typically,the layer can comprise approximately 50 to 97 Mol % ZrO₂, CeO₂, HfO₂ orThO₂, and 50 to 3 Mol % CaO, MgO or SrO, and/or oxides of rare earthsand in particular Y₂ O₃. Advantageously, the layer comprises ZrO₂stabilized with Y₂ O₃. The thickness of the layer can advantageouslyrange from 10 to 200 micrometers, in particular 15 to 50 micrometers.

The pastes used to produce the solid electrolyte layer can be producedusing binders and/or adhesion promoters, plasticizers and organicsolvents.

The insulation layer 10, which insulates the conductor track 3' of theouter pump electrode 3 from the solid electrolyte layer 9, comprises aninsulating layer, for instance based on Al₂ O₃, such as that typicallyproduced in the production of planar sensor elements, for insulatingconductor tracks from a solid electrolyte. The insulation layer 10 mayfor instance be 15 to 20 micrometers thick.

Optionally, such an insulation layer can also be disposed between thesubstrate 1 and the conductor track 2' of the inner pump electrode 2,for instance in the case in which the substrate is asolid-electrolyte-based substrate, such as a ZrO₂ substrate. Thedisposition of such insulation layers is not absolutely necessary,however.

The engobe 11 is porous and for instance comprises a layer based on Al₂O₃ or Mg-spinel, as is typically used in planar sensor elements to coverelectrodes. The thickness of the engobe is for instance in the rangefrom 10 to 40 micrometers.

In an advantageous embodiment of the invention, the porous engobecomprises an Al₂ O₃ and/or Mg-spinel matrix, with ZrO₂ particlesembedded in it, of the type known from German Patent Disclosure Document37 37 215.

EXAMPLE

To produce a sensor element of the type schematically shown in FIG. 3, afoil of zirconium dioxide, stabilized with yttrium, having a layerthickness of 0.5 mm, was used as the substrate. The diffusion layer 5 inthe geometrical shape shown in FIG. 2 was incorporated by thick-filmtechnology by means of a screenprinted layer of a mixture of theobromineand coarse-grained ZrO₂ having a particle size of 10 micrometers; in thelater sintering process in the temperature range of about 300° C., thetheobromine evaporated. The ZrO₂ solid electrolyte layer 9 was producedby printing with a paste of ZrO₂ stabilized with Y₂ O₃, having aparticle size of approximately 1 to 2 micrometers. The printed layer hada thickness of 80 micrometers. The pump electrodes 2 and 3, comprisingplatinum, were also applied by a known screenprinting technique; a10-micrometers-thick Al₂ O₃ isolation layer was applied beforehand tothe surface of the solid electrolyte layer carrying the outer pumpelectrode, in the region of the conductor track of the outer pumpelectrode. The pump electrodes had a thickness of 12 micrometers. Theconductor tracks were produced based on a typical Pt cermet pastecomprising 85 part by weight of Pt powder and 15 parts by weight of YSZpowder.

To produce the engobe 11, a paste based on Al₂ O₃ was printed on. Theengobe had a thickness of approximately 30 micrometers.

The cover layer 7 was likewise printed, beginning with a paste based onAl₂ O₃. It had a thickness of approximately 10 micrometers.

After the application of the electrodes, conductor tracks, isolationlayer and engobe and cover layer, the coated substrate was subjected toa sintering process, in which it was heated for approximately 3 hours toa temperature in the range of 1380° C.

After suitable calibration by severing zigzag segments, the sensorelement produced was inserted into a housing of the type known fromGerman Patent Disclosure Document 32 06 903 and used to determine thelambda value of gas mixtures. Excellently reproducible results wereobtained.

The production of a sensor element according to the invention ispreferably done by machine using the multiple printed panel technique.Advantageously, the width of the sensor is approximately 4 to 6 mm. Theelectrode diameter is advantageously from 3 to 4 mm, for instance 3.6mm.

We claim:
 1. Sensor element for limiting-current sensors to determinethe lambda value of gas mixtures, in particular of exhaust gases ofinternal combustion engines, havinga solid electrolyte ceramic substrate(1), first and second pump electrodes (2, 3) disposed on opposite sidesof said solid electrolyte ceramic substrate (1), said first pumpelectrode (2) being accessible for a measurement gas, said gas beingsupplied to said first pump electrode through a diffusion layer (5)acting as a diffusion barrier and located on said solid electrolyteceramic substrate adjacent to said first pump electrode; said first pumpelectrode and said diffusion layer having a gas-tight cover layer (7)covering said first pump electrode (2) and the diffusion layer (5) forexclusion of the measurement gas, except for a measurement gas inletopening (4) located at an edge of said diffusion layer remote from andopposite an edge thereof that adjoins said first pump electrode; a firstconductor track (2') on said substrate (1) for the first pump electrode(2), said first conductor track being covered by said gas-tight coverlayer (7), and a second conductor track (3') on said substrate (1) forsaid second pump electrode (3), characterized in that the diffusionlayer (5) has, at its edge which is nearest to the measurement gas inletopening (4), a series of outwardly tapered projections, each orientedtowards said measurement gas inlet opening, said gas-tight layer fillingall gaps between said projections, whereby the finished sensor elementis calibratable by at least one mechanical or laser cutting operationwhich cuts off extremity portions of a plurality of said projections andthereby reduces the diffusion resistance of the diffusion layer.
 2. Thesensor element of claim 1, characterized in thatsaid tapered projectionof said edge of said diffusion layer (5) together present azigzag-contoured edge having equally spaced edge extremities open oropenable for producing said measurement gas inlet opening (4) and inthat the number and spacing from one another of the edge extremities ofthe zigzag-contoured edge and the length of said tapered projections areselected to optimize the load of said first pump electrode.
 3. Thesensor element of claim 2,characterized in that the diffusion layer (5),including said zigzag-profiled edge portion thereof, is printed onto thesolid electrolyte ceramic substrate (1) by a screenprinting process. 4.The sensor element of claim 2,characterized in that the solidelectrolyte ceramic substrate (1) comprises ZrO₂ stabilized with Y₂ O₃.5. The sensor element of claim 1,characterized in that the diffusionlayer (5) comprises a porous layer consisting essentially of a materialselected from the group consisting of Al₂ O₃ and ZrO₂.
 6. The sensorelement of claim 5,characterized in that the solid electrolyte ceramicsubstrate (1) comprises ZrO₂ stabilized with Y₂ O₃.
 7. The sensorelement of claim 1,characterized in that the diffusion layer (5),including said outwardly tapered edge projections thereof, is printedonto the solid elect ceramic substrate (1) by a screenprinting process.8. The sensor element of claim 7, characterized in thatthe solidelectrolyte ceramic substrate (91) comprises ZrO₂ stabilized with Y₂ O₃.9. The sensor element of claim 1,characterized in that the solidelectrolyte ceramic substrate (1) comprises ZrO₂ stabilized with Y₂ O₃.10. A sensor element for limiting-current sensors for determining thelambda value of gas mixtures, in particular of exhaust gases of internalcombustion engines, comprising:a ceramic substrate (1), a diffusionlayer (5) on one side of said substrate (1) for producing a permeablediffusion barrier for reducing diffusion of a measurement gas, saiddiffusion layer (5) covering less than all of said side of saidsubstrate, an inner pump electrode (2) on a portion of said diffusionlayer and having a first conductor track (2') on a portion of said onesubstrate side which is not covered by said diffusion layer, a solidelectrolyte gas-tight cover layer (9) directly covering said inner pumpelectrode (2) and portions of said diffusion layer (5) not covered bysaid inner pump electrode (2) and conformably covering said diffusionlayer where it would otherwise be exposed to measurement gas except fora measurement gas inlet opening (12) located at an edge portion of saiddiffusion layer (5) which is opposite and remote from said innerelectrode, an outer pump electrode (3) on said solid electrolytegas-tight cover layer (9) connected to a second conductor track (3'); aninsulating layer (10) on said solid electrolyte gas-tight cover layer 9and interposed between said solid electrolyte gas-tight cover layer (9)and said first conductor track (2'), whereby said second conductor track(3') is insulated from said first conductor track (2'), and a porousengobe layer (11) on said outer pump electrode (3), characterized inthat said diffusion layer (5) has at the edge thereof at which saidmeasurement gas inlet opening (12) is located, a series of outwardlytapered projections each oriented towards the measurement gas inletopening (12), said solid electrolyte gas-tight cover layer (9) fillingall gaps between said projections whereby the finished sensor element iscalibratable by at least one mechanical or laser cutting operation whichcuts off extremity portions of a plurality of said outwardly taperedprojections and thereby reduces the diffusion resistance of thediffusion layer.
 11. The sensor element of claim 10, wherein a gas-tightlayer (7) of Al₂ O₃ base is provided covering said second conductortrack (3') and wherein said solid electrolyte cover layer (9) extendsabove said first conductor track (2') beneath said insulating layer(10).
 12. The sensor element of claim 10, wherein the presence of saidprojections provides a configuration of said edge of said diffusionlayer remote from said inner pump electrode which has a zigzag-profileand wherein said projections have equally spaced extremities.
 13. Thesensor element of claim 12,characterized in that the diffusion layer (5)comprises a porous layer consisting essentially of a material selectedfrom the group consisting of Al₂ O₃ and ZrO₂.
 14. The sensor element ofclaim 12,characterized in that the diffusion layer (5), including saidtapered projections, is printed into the ceramic substrate (1) by ascreening process.
 15. The sensor element of claim 12,characterized inthat the ceramic substrate (1) comprises ZrO₂ stabilized with Y₂ O₃. 16.The sensor element of claim 10, characterized in that the diffusionlayer (5) comprises a porous layer consisting essentially of a materialselected from the group consisting of Al₂ O₃ and ZrO₂.
 17. The sensorelement of claim 10,characterized in that the diffusion layer (5),including said tapered projections, is printed onto the ceramicsubstrate (1) by a screenprinting process.
 18. The sensor element ofclaim 10,characterized in that the ceramic substrate (1) comprises ZrO₂stabilized with Y₂ O₃.